ABSTRACT A fundamental question in musculoskeletal development, repair and regeneration is how mesenchymal stem/stromal cells regulate development and repair processes. Our recently published data shows that, in the skeleton, Hox genes are exclusively expressed in progenitor-enriched, bone marrow mesenchymal stem/stromal cells (BM-MSCs) and Hox11 gene expression is confined to the zeugopod limb region (radius/ulna; tibia/fibula). Loss of Hox function leads to defects in bone repair in adults as well as previously identified defects in the embryonic skeleton (Rux, et al, in press, Developmental Cell). We have also previously shown that Hox11 genes are expressed in muscle connective tissue stromal cells with the same regional restriction, and function at developmental stages to pattern the muscles of the limb. New preliminary data shows that muscle stromal expression continues through adult stages and expands in response to muscle injury, consistent with continued function in adult muscle tissue. We have generated a Hoxd11 conditional allele that will allow us to selectively remove Hox11 function at postnatal or adult stages and examine continued function at adult stages. In both the skeletal and muscle tissue, Hox genes are expressed only in the stromal progenitor cells (BM-MSCs in the skeleton) that are known to be important for proper developmental patterning (as well as repair in the skeleton) and both cell populations show regional specificity. Our Hoxa11eGFP knock-in reporter allows us to probe the biological similarities and differences between these two stromal populations as well as between controls and mutants at developmental stages and during injury repair/regeneration. The goal of this study is to investigate the continued role for Hox genes in muscle stromal cells and the nature of the signaling pathways regulated in muscle stromal cells during development and repair. A successful outcome from these analyses will demonstrate a novel function for Hox genes in adult muscle repair and will provide critical new information regarding the nature of connective tissue stromal cell regulation of muscle development and repair processes and how these stromal progenitors compare to more well-characterized skeletal MSCs. The data generated through this study will provide the basis for continued funding on this important aspect of muscle and MSC biology.